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CHAPTER23 THE EVOLUTION OF POPULATIONS OUTLINE I. PopulationGenetics A. The modern evolutionary synthesisintegratedDarwinianselection and Mendelian inheritance B. The genetic structure of a population is defined by its allele and genotype frequencies theoremdescribesa nonevolvingpopulation C. The Hardy-Weinberg II. Causesof Microevolution changein a population'sallele or A. Microevolutionis a generation-to-generation genotypefrequencies B. The five causesof microevolution are genetic drift, gene flow, mutation, nonrandommating, and natural selection III. GeneticVariation,the Substratefor Natural Selection A. Geneticvariationoccurswithin and betweenpopulations B. Mutation and sexualrecombinationgenerategeneticvariation C. Diploidy and balancedpolymorphismpreservevariation IV. Natural Selectionas the Mechanismsof AdaptiveEvolution A. Evolutionary fitness is the relative contribution an individual makes to the gene pool of the next generation B. The effect of selectionon a varying characteristiccan be stabilizing,directional,or diversiffing C. Sexualselectionmay leadto pronouncedsecondarydifferencesbetweenthe sexes D. Natural selectioncannotfashionperfectorganisms OBJECTIVES After readingthis chapterand attendinglecture,the studentshouldbe ableto: "modernsynthesis"' 1. Explainwhat is meantby the Explain how microevolutionarychangecan affect a genepool. 2. In their own words, statethe Hardy-Weinbergtheorem' 3. Write the generalHardy-Weinbergequationand useit to calculateallele and genotype 4. frequencies. of Hardy-Weinbergequilibrium' Explain the consequences 5. Demonstrate,with a simpleexample,that a disequilibriumpopulationrequiresonly one 6. generationof randommiting to establishHardy-Weinbergequilibrium. Describethe usefulnessof the Hardy-Weinbergmodelto populationgeneticists. 7. 340 Unit IV Mechanismsof Evolution 8. 9. 10. I l. 12. 13. 14. I 5. 16. 17. 18. 19. 20. 21. 22. 23. 24. List the conditions a population must meet in order to maintain Hardy-Weinberg equilibrium. Explain how genetic drift, gene flow, mutation, nonrandom mating and natural selection can causemicroevolution. Explain the role of population size in genetic drift. Distinguish between the bottleneckeffect and the founder effect. Explain why mutation has little quantitative effect on a large population. Describehow inbreedingand assortative mating affect a population's allele frequencies and genotype frequencies. Explain, in their own words, what is meant by the statement that natural selection is the only agent of microevolution which is adaptive. Describe the techniqueof electrophoresisand explain how it has been usedto measure genetic variation within and betweenpopulations. List some factors that can produce geographical variation among closely related populations. Explain why even though mutation can be a source of genetic variability, it contributes a negligible amount to genetic variation in a population. Give the causeof nearly all genetic variation in a population. Explain how genetic variation may be preservedin a natural population. In their own words, briefly describe the neutral theory of molecular evolution and explain how changes in gene frequency may be nonadaptive. Explain what is meant by "selfish" DNA. Explain the concept of relative fitness and its role in adaptive evolution. Explain why the rate of decline for a deleteriousallele dependsupon whether the allele is dominant or recessiveto the more successfulallele. Describewhat selection acts on and what factors contribute to the overall fitness of a genotype. 25. 26. Give examplesof how an organism'sphenotypemay be influenced by the environment. Distinguish among stabilizing selection,directional selectionand diversiffing selection. 27. Define sexual dimorphism and explain how it can influence evolutionary change. Give at least four reasonswhy natural selection cannot breed perfect organisms. 28. KEYTERMS population genetics modern synthesis population species gene pool genetic structure microevolution bottleneck effect geographicalvariation cline founder effect geneflow balancedpolymorphism mutation inbreeding Hardy-Weinberg assortativemating theorem,equilibrium, naturalselection and equation polymorphism heterozygote advantage hybrid vigor frequency-dependent selection neutralvariation Darwinian fitness relative fitness stabilizing selection directional selection diversifuing selection sexual dimorphism sexual selection LECTT]RENOTES Natural selectionworks on individuals, but it is the population that evolves (see Campbell Figure 23.1). Darwin understoodthis, but was unableto determineits eeneticbasis. Chapta23 TheEvolutionofPoptrlations 34I I. Population Genetics A. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance Shortly after the publication of The Origin of Species,most biologists. were convinced that speciesevolved. Darwin was less successful in convincing them that natural selection was the mechanism for evolution, becauselittle was known about inheritance' . An understanding about inheritance was necessaryto explain how: + Chancevariations arise in populations =+ These variations are precisely transmitted from parents to offspring . Though Gregor Mendel was a contemporaryof Darwin's, Mendel's principles of inheritancewent unnoticed until the early 1900s' For Darwin, the raw material for natural selection was variation in quantitative characters that vary along a continuum in a population' . We now know that continuous variation is usually determined by many segregatingloci (polygenic inheritance). . Mendel and geneticistsin the early 1900s recognized only discrete characters inherited on an either-or basis. Thus, for them, there appearedto be no genetic basis for the subtle variations that were central to Darwin's theory. In the 1930s,the scienceof populotion genetics emerged,which: . Emphasized genetic variation within populations and recognized the importance of quantitative characters . Was an important turning point for evolutionary theory, becauseit reconciled Mendelian geneticswith Darwinian evolution In the 1940s, the genetic basis of variation and natural selection was worked out, and the modern synthesis was formulated. This comprehensive theory: . Integrated discoveries from different fields (e.g., paleontology, taxonomy, biogeography,and population genetics) ' Was collectively developedby many scientistsincluding: + TheodosiusDobzhansky* geneticist = + = . Ernst Mayr - biogeographer and systematist George Gaylord Simpson- paleontologist G. LedYard Stebbins- botanist Emphasizedthe following: =+ Importance of populations as units of evolution =+ The central role of natural selection as the primary mechanism of evolutionary change = Gradualism as the explanation of how large changes can result from an accumulationof smalf changesoccurring over long periods of time Most of Darwin's ideas persisted in the modern synthesis, although many evolutionary biologists are challenging some generalizationsof the modern synthesis. . This debatefocuseson the rate of evolution and on the relative importance of evolutionary mechanismsother than natural selection' . These debatesdo not questionthe fact of evolution, only what mechanismsare most imPortant in the Process. . Such disagreementsindicate that the study of evolution is very lively and that it continuesto develoPas a science. 342 Unit IV Mechanisms of Evolution B . The genetic structure of a population is defined by its allele and genotype frequencies Population = Localized group of organisms which belong to the same species Species=A group of populationswhose individuals have the potential to interbreedand produce fertile offspring in nature Most species are not evenly distributed over a geographical range, but are concentrated in several localized population centers. ' Each population center is isolated to some extent from other population centerswith only occasionalgene flow among thesegroups. ' Obvious examplesare isolatedpopulations found on widely separatedislandsor in unconnectedlakes. . Some populationsare not separatedby such sharpboundaries. + For example, a specieswith two population centersmay be connectedby an intermediatesparselypopulatedrange. =t Even though these two populations are not absolutely isolated, individuals are more likely to interbreedwith others from their population center (see Campbell, Figure 23.2). Gene flow between the two population centers is thus reducedby the intermediaterange. Genepool : The total aggregateof genesin a population at any one time ' Consists of all the alleles at all gene loci in all individuals of a population. Alleles from this pool will be combined to produce the next generation, ' In a diploid species,an individual may be homozygous or heterozygous for a locus since each locus is representedtwice. ' An allele is said to befixed in the gene pool if all membersof the population are homozygous for that allele. ' Normally there will be two or more alleles for a gene, each having a relative frequency in the gene pool. c . The Hardy-Weinberg theorem describes a nonevolving population The Hardy- Weinberg model is much easier to teach if the students calculate gene frequenciesalong with the instructor. This means that you must pause frequently to allow plenty of time for studentsto actively processthe information and prictice the calculations. In the absenceof other factors, the segregationand recombinationof alleles during meiosis and fertilization will not alter the overall genetic makeup of a population. ' The frequenciesof alleles in the gene pool will remain constant unless acted upon by other agents;this is known as the Hardy-Weinberg theorem. ' The Hardy-Weinberg model describesthe genetic structure of nonevolving populations. This theorem can be tested with theoretical population models. To test the Hardy-Weinberg theorem, imagine an isolatedpopulation of wildflowers with the following characteristics(see Campbell, Figure 23.3): . It is a diploid specieswith both pink and white flowers. ' The population size is 500 plants: 480 plants have pink flowers, 20 plants have white flowers. ' Pink flower color is coded for by the dominant allele "A," white flower color is coded for by the recessiveallele "a." ' of the 480 pink-flowered plants, 320 are homozygous (AA) and 160 are heterozygous(Aa). Sincewhite color is recessive,all white flowered plants are homozygous aa. Chryter23 TheEvolutionofPopuldions343 There are 1000 genesfor flower color in this population, since each of the 500 individuals has two genes(this is a diploid species). A total of 320 genes are present in the 160 heterozygotes (Aa): half are dominant (160 A) and half are recessive(160 a). 800 of the 1000 total genes are dominant. The frequency of the A allele is 80% or 0.8 (800/1000). a a Genotypes # of # of A alleles Total # plants per individual A alleles AA plants 320 Aa plants 160 x 2 640 160 200 ofthe 1000total genesare recessive. The frequencyof the a allele is20% or 0.2 (200/1000). a a Genotypes # of plants aa plants 20 Aa plants 160 # of A alleles Total # per individual A alleles x 2 x 1 40 160 Assuming that mating in the population is completely random (all male-female mating combinations have equal chances),the frequencies of A and a will remain the same in the next generation. . Each gamete will carry one gene for flower color, either A or a' . Since mating is random, there is an 80Yochance that any particular gamete will carry the e allele and a 20o/ochance that any particular gamete will carry the a allele. The frequencies of the three possible genotypes of the next generation can be calculatedusing the rule of multiplication (see Campbell, Chapter l4): : . The probability of two A allelesjoining is 0.8 x 0.8 0.64; thus, 64Yoof the . . next generation will be AA. The probability of two a allelesjoining is 0.2 x 0.2 = 0.04; thts, 4Yoof the next generationwill be aa. Heterozygotes can be produced in two ways, depending upon whether the sperm or ovum contains th-e dominant allele (Aa or aA). The probability of a = heterozygotebeing producedis thus (0.8 x 0.2) + (0.2 x 0.8) = 0.16 + 0.16 0.32. The frequencies of possible genotypes in the next generation are 64Yo AA,32yo Aa and 4oh aa. 344 Unit IV Mechanismsof Evolution ' Th9 frequency of the A allele in the new generation is 0.64 + (0.3212) : 0.g, and the frequencyof the a allele is 0.04 + (0.3212) :0.2. Note that the alleles are presentin,the gene pool of the new population at the same freqtencies they were in the original gene pool. ' Continued sexual reproduction with segregation,recombination and random mating would not alter the frequenciesof thesetwo alleles:the gene pool of this population would be in a state of equilibrium referred to as Hardy-Weinberg equilibrium. ' If our original population had not been in equilibrium, only one generation would have been necessaryfor equilibrium to becomeestablished. From this theoretical wildflower population, a general formula, called the HardyIleinberg equation, can be derived to calculate allele and genotype frequencies. ' The Hardy-Weinberg equation can be usedto consider loci with three or more alleles. ' By way of example, consider the simplest casewith only two alleles with one dominant to the other. ' In our wildflower population, let p represent allele A and q represent allele a, thusp : 0.8 and q= 0.2. ' The sum of frequenciesfrom all alleles must equal 100% of the genesfor that locus in the population:p + q : 1. ' Where only two alleles exist, only the frequency of one must be known since the other can be derived: l-p:q or 1-q:p when gametes fuse to form a zygote, the probability of producing the AA genotype is p'; the probability of producing aa is q'; and the probability of producing an Aa heterozygote is 2pq (remember heterozygotesmay be formed in two ways). . The sum of thesefrequenciesmust equal 100%, thus: p2 Frequency ofAA + 2 p q Frequency of Aa + q 2 I Frequency of aa The Hardy-Weinberg equationpermitsthe calculationof allelic frequencies in a gene pool, if the genotypefrequencies are known.Conversely, the genotypecan be calculatedfrom knownallelicfrequencies. For example,the Hardy-Weinbergequationcan be usedto calculatethe frequencyof inheriteddiseases in humans(e.g.,phenylketonuria): . 1 of every 10,000 babiesin the United Statesis born with phenylketonuria (PKU), a metabolic disorderthat, if left untreated,can result in mental retardation. ' The allelefor PKU is recessive,so babieswith this disorderare homozysous : q2. recessive . Thusq': O.OOO1, with q:0.01 (thesquare rootof 0.0001). . The frequencyof p can be determinedsincep : 1 - q: p : 1 - 0 . 0 1: 0 ' 9 9 . The frequencyof carriers(heterozygotes) in the populationis 2pq. . 2 p q = 2 ( 0 . 9 9 X 0 . 0=1 )0 . 0 1 9 8 Thus, about2Yoof the U.S. populationare carriersfor PKU. Chapta23 TheEvolutionofPopulation" 345 II. Causes of Microevolution A. Microevolution is a generation-to-generation change in a population's allele or genotype frequencies The Hardy-Weinbergequilibrium is important to the study of evolution since it tells us what will happen in a nonevolving population. . This equilibrium model providesa baseline from which evolutionary departures take place. . It provides a reference point with which to compare the frequenciesof alleles and genotypesof natural populationswhose gene pools may be changing. If the frequencies of alleles or genotypes deviate from values expected from the HardyWeinberg equilibrium, then the population is evolving . Therefore, a refined definition of evolution at the population level is a generation-to-generation change in a population's frequencies of alleles or genotYPes. . Becausesuch change in a gene pool is evolving on the smallest scale, it is referred to as microevolution. B. The five causes of microevolution are genetic drift, gene flow, mutation' nonrandom mating, and natural selection For Hardy-Weinbergequilibrium to be maintained,five conditions must be met: Very large poPulationsize Z. Isolation from other populations. There is no migration of individuals into or out of the PoPulation. 3. No net mutations 4. Random mating 5. No natural selection. All genotypes are equal in survival and reproductive success.Differential reproductivesuccesscan alter gene frequencies. In real populations, several factors can upset Hardy-Weinberg equilibrium and cause microevolut ionary change. Microevolution : Small scale evolutionary change representedby a generational shift in a population's relative allelic frequencies. . Microevolution can be causedby genetic drift, geneflow, mutation, nonrandom mating,andnatural selection;each of these conditions is a deviation from the criteria for Hardy-Weinbergequilibrium. . Of these five possible agents for microevolution, only natural selection generally leads tb an accumulationof favorable adaptationsin a population. . The other four are nonadaptive and are usually called non-Darwinian changes. l. 1. Genetic drift o':"",1';o*rff l,::'I'*"T::,1":*ilJ::1"'"r'":"T:r'T"i;:T repreienied in the next generationbecauseof sampling error' Chance eventsmay causethe frequenciesof alleles to drift randomly from generation to generation, since the existing gene pool may not be accuratelyrepresentedin the next generation. For example, assumeour theoreticalwildflower population contains_only 25 plants, and the ginotypes for flower color occur in the following numbers: 16 AA, 8 Aa and I aal tn tiris case, a chance event could easily change the frequencies of the two alleles for flower color (see Campbell,Figure 23'4)' 346 Unit IV- Mechanisms of Evolution o I rock slide or passing herbivore which destroys three AA plants would immediatelychangethe frequenciesof the allelesfrom A:0.8 and a= 0.2 t o A : 0 . 7 7 a n da : 0 . 2 3 . ' Although this changedoes not seemvery drastic,the frequenciesof the two alleles were changedby a chanceevent. The larger the population, the less important is the effect of genetic drift. ' Even though natural populations are not infinitely large (in which case genetic drift could be completely eliminated as a causeof microevolution), most are so large that the effect of genetic drift is negligible. . However, some populations are small enough that genetic drift can play a major role in microevolution, especiallywhen the population has less than 100 individuals. Two situationswhich result in populationssmall enough for genetic drift to be important are the bottleneck ffict andthefounder ffict. a. The bottleneck effect The size of a population may be reduceddrasticallyby such natural disastersas volcanic eruptions,earthquakes,fires, floods, etc. which kill organisms nonselectively. ' The small surviving population is unlikely to represent the genetic makeup of the original population. . Genetic drift which results from drastic reduction in population size is referred to as the bottleneck effect. ' By chancesome individuals survive. In the small remaining population, some allelesmay be overrepresented,some underrepresented,and some alleles may be totally absent(see Campbell, Figure 23.5). ' Genetic drift which has occurredmay continue to affect the population for many generations, until it is large enough for random drift to be insignificant. The bottleneck effect reducesoverall genetic variability in a population since some alleles may be entirely absent. . For example, a population of northern elephant seals was reducedto just 20 individuals by huntersin the 1890's. + Since this time, these animals have been protected and the population has increasedto about 30,000 animals. = Researchershave found that no variation exists in the 24 loci examined from the present population. A single allele has been fixed at each of the 24 loci due to genetic drift by the bottleneck effect. + This contrasts sharply with the large amount of genetic variation found in southernelephant seal populations which did not undergo the bottleneck effect. o d lack of genetic variation in South African cheetahs may also have resulted from genetic drift, since the large population was severely reducedduring the last ice age and again by hunting to near extinction. b. The founder effect When a few individuals colonize a new habitat,genetic drift is also likely to occur. Genetic drift in a new colony is called thefounder effect. . The smaller the founding population,the less likely its gene pool will be representativeof the original population'sgenetic makeup. . The most extreme example would be when a single seed or pregnant female moves into a new habitat. Chqter 23 TheEvolutionofPopulations 347 . If the new colony survives, random drift will continue to affect allele frequencies until the population reaches a large enough size for its influenceto be negligible. . No doubt, the founder effect was instrumental in the evolutionary divergenceof the Galapagosfinches. The founder effect probably resulted in the high frequency of retinitis pigmentosa (a progressive form of blindness that affects humans homozygous for this recessive allele) in the human population of Tristan da Cunha, a group of small Atlantic islands. . This areawas colonizedby 15 people in 1814, and one must have been a carrier. . The frequency of this allele is much higher on this island than in the populations from which the colonists came. Although inherited diseasesare obvious examplesof the founder effect, this form of genetic drift can alter the frequencies of any alleles in the gene pool. 2. Gene flow Geneflow: The migration of fertile individuals, or the transfer of gametes, between populations . Natural populations may gain or lose alleles by gene flow, since they do not have gene pools which are closed systems required for Hardy-Weinberg equilibrium. . Gene flow tends to reduce between-population differences which have accumulated by natural selection or genetic drift. . An example of gene flow would be if our theoretical wildflower population was to begin receiving wind blown pollen from an all white-flower population in a neighboringfield. This new pollen could greatly increasethe frequency of the white flower allele, thus also altering the frequency of the red flower allele. . Extensive gene flow can eventually group neighboring populations into a single population. 3. Mutations A new mutation that is transmitted in gametes immediately changes the gene pool of a population by substituting one allele for another. In our theoretical wildflower population, if a mutation in a white flowered plant causedthat plant to begin producing gameteswhich carried a red flower allele, the frequency of the white flower allele is reduced and the frequency of the red flower allele is increased. Mutation itself has little quantitative effect on large populations in a single generation, since mutation at any given locus is very rare. . Mutation rates of one mutation per 105to 106 gametes are typical, but vary dependingon the speciesand locus. . An allele with a 0.50 frequency in the gene pool that mutates to another allele at a rate of 10-' mutations per generation would take 2000 generationsto reduce the frequency of the original allele from 0.50 to 0.49. . The gene pool is effected even less, since most mutations are reversible. . If a new mutation increasesin frequency, it is becauseindividuals carrying this allele are producing a larger percentageof offspring in the population due to genetic drift or natural selection, not becausemutation is producing the allele in abundance. 348 Unit IV Mechanisms of Evolution Mutation is important to evolution since it is the original sourceof genetic variation, which is the raw material for natural selection. 4. Nonrandom mating Nonrandom mating increasesthe number of homozygousloci in a population, but does not in itself alter frequenciesof alleles in a population'sgene pool. There are two kinds of nonrandom mating: inbreeding and assortstive mating. a. Inbreeding Individuals of a population usually mate with close neighborsrather than with more distant membersof a population, especially if the membersof the population do not dispersewidely. . This violates the Hardy-Weinberg criteria that an individual must choose its mate at random from the population. . Since neighboring individuals of a large population tend to be closely related, inbreedingis promoted. . Self-fertilization, which is common in plants, is the most extreme example of inbreeding. Inbreeding results in relative genotypic frequencies(pt,2pq, q21that deviate from the frequenciespredictedfor Hardy-Weinbergequilibrium, but does not in itself alter frequenciesof alleles (p and q) in the gene pool. Self-fertilization in our theoreticalwildflower population would increasethe frequenciesof homozygousindividuals and reducethe frequencyof heterozygotes. . Selfing of AA and aa individualswould producehomozygousplants. . Selfing of Aa plants would produce half homozygotes and half heterozygotes. . Each new generation would see the proportion of heterozygotes decrease, while the proportions of homozygous dominant and homozygousrecessiveplants would increase. . Inbreeding without selfing would also result in a reduction of heterozygotes,although it would take much longer. One effect of inbreeding is that the frequency of homozygousrecessive phenotypes increases. An interesting thing to note is that even if the phenotypic and genotypic ratios change,the values of p and q do not changein these situations,only the way they are combined. A smaller proportion of recessivealleles are maskedby the heterozygous state. b. Assortative mating Assortative mating is another type of nonrandom mating which results when individuals mate with partnersthat are like themselvesin certain phenotypic characters. Examples: . Some toads (Bufo) most commonly mate with individualsof the same size (see Campbell,Figure 23.6). . Snow geeseoccur in a blue variety and a white variety, with the blue color allele being dominant. Birds prefer to mate with those of their own color, blue with blue and white with white; this results in a lower frequency of heterozygotes than predicted by Hardy-Weinberg. . Blister beetles (Lytla magister) in the Sonoran Desert usually mate with a samesize individual. Chpter 23 TheEvolutionofPopulations 349 5. Natural selection The Hardy-Weinbergequilibrium condition that all individuals in a population have *":'"TT*l.^:ffi :fril'lil'"'#ff :y;:LT:'vidua,sex Hl:,:"'ffi and some variants leave more offspring than others. . Natural selection is this differential successin reproduction. Due to selection,alleles are passedon to the next generationin disproportionate "-:"",i:1":T:;: jiilf'*i,:HT.l":iiffi1ffi H:,,u."*o,"visib,e plants with pink flowers (both AA and Aa) pink flowers, herbivores than on the average. leave more offspring would Genetic equilibrium would be disturbed and the frequency of allele A would increaseand the frequencyof the a allele would decrease. Natural selectionis the only agent of microevolution which is adaptive, since it *":-'3:;::1T,Ti:nffi 'L'ectionravoringsenotypesp :,'li"ll'ff in the population which can survive the new conditions. Variability in the population makes it possible for natural selection to occur. Itr. Genetic Variation, the Substrate for Natural Selection Members of a population may vary in subtle or obvious ways. It is the genetic basis of this variation that makes natural selectionpossible' A. Genetic variation occurs within and between populations Darwin consideredthe slight differences between individuals of a population as raw material for natural selection. While we are more consciousof the variation among humans,an equal if not greater amount of variation exists among the many plant and animals species. . Phenotypic variation is a product of inherited genotype and numerous environmental influences. . Only the genetic or inheritable component of variation can have adaptive impact as a result of natural selection. 1. Variation within PoPulations Both quantitative and discrete characters contribute to variation within a o"o:t";;ygenic characters,which vary quantitatively within a population, are responsiblefor much of the inheritable variation. . For example, the height of the individuals in our theoretical wildflower population may vary from very short to very tall with all sorts of intermediateheights. Discrete characters,such as pink vs. white flowers, can be classifiedon an either-or basis, usually becausethey are determined by only one locus with different alleles that produce distinct phenotypes' ^. PolymorPhism In our wildflower population, the red and white flowers would be referred to as different morphs (contrasting forms of a Mendelian character). A population is referred to as polymorphic for a character if two or more morphs are presentin noticeablefrequencies(see Campbell, Figure 23.7). 350 Unit IV Mechanisms of Evolution Polymorphism is found in human populationsnot only in physical characters (e.g., presenceor absenceof freckles) but also in biochemical characters(e.g., ABO blood group). b. Measuring genetic variation Darwin did not realize the extent of genetic variation in populations, since much of the genetic variation can only be determinedwith biochemical methods. . Electrophoresis has been used to determine genetic variation among individuals of a population.This techniqueallows researchersto identifu variations in protein products of specific gene loci. . Electrophoretic studies show that in Drosophila populations the gene pool has two or more alleles for about 30% of the loci examined, and each fly is heterozygousat about l2o/oof its loci. . Thus, there are about 700 - 1200 heterozygous loci in each fly. Any two flies in a Drosophila population will differ in genotype at about 25% of their loci. . Electrophoretic studies also show comparable variation in the human population. . Note that electrophoresisunderestimatesgenetic variation: . Proteins produced by different alleles may vary in amino acid composition and still have the same overall charge, which makes them indistinguishableby electrophoresis. . Also, DNA variation not expressed as protein is not detected by electrophoresis. 2. Variation between populations Geographical variation in allele frequencies exists among populations of most species. . Natural selection can contribute to geographicalvariation, since at least some environmental factors are different between two locales. For example, one population of our wildflowers may have a higher frequency of white flowers becauseof the prevalence in that area of pollinators that prefer white flowers. . Genetic drift may cause chance variations among different populations. . Also, subpopulationsmay appear within a population due to localized inbreeding resulting from a "patchy" environment. Cline : One type of geographical variation that is a graded change in some trait along a geographic transect . Clines may result from a gradation in some environmental variable. . It may be a graded region of overlap where individuals of neighboring populations interbreed. . For example, the average body size of many North American mammal species gradually increaseswith increasing latitude. It is presumed that the reduced surface area to volume ratio associated with larger size helps animals in cold environmentsconservebody heat. . Studies of geographical variation confirm that genetic variation affects spatial differences of phenotypes in some clines. For example, yarrow plants are shorter at higher elevations, and some of this phenotypic variation has a geneticbasis (seeCampbell, Figure 23.8) Chapta23 TheEvolutionofPopulations351 B. Mutation and sexual recombination generate genetic variation Two random processes,mutation and sexual recombination(see Chapter l5), create variation in the genetic composition of a population. 1. Mutation Mutations produce new alleles. They are rare and random events which usually occur in somatic cells and are thus not inheritable. . Only mutations that occur in cell lines which will produce gametes can be passedto the next generation. . Geneticists estimate that only an average of one or two mutations occur in each human gamete-producingcell line. Point mutation : Mutation affecting a single base in DNA . Much of the DNA in eukaryotes does not code for proteins, and it is uncertain how a point mutation in these regions affect an organism. . Point mutations in structural genes may cause little effect, partly due to the redundancyofthe genetic code. Mutations that alter a protein enough to affect the function are more often harmful than beneficial, since organisms are evolved products shaped by selection and a chance change is unlikely to improve the genome. . Occasionally, a mutant allele is beneficial, which is more probable when environmental conditions are changing. . The mutation which allowed house flies to be resistant to DDT was present in the population and under normal conditions resulted in reduced growth rate. It became beneficial to the house fly population only after a new environmental factor (DDT) was introduced and tipped the balance in favor of the mutant alleles. Chromosomal mutations usually affect many gene loci and tend to disrupt an organism's development. . On rare occasions,chromosomal rearrangement may be beneficial. These instances (usually by translocation) may produce a cluster of genes with cooperative functions when inherited together. Duplication of chromosomesegmentsis nearly always deleterious. . If the repeated segment does not severely disrupt genetic balance, it may persist for several generations and provide an expanded genome with extra loci. . These extra loci may take on new functions by mutation while the original genes continue to function. . Shuffling of exons within the genome (single locus or between loci) may also produce new genes. Mutation can produce adequategenetic variation in bacteria and other microorganismswhich have short generationtimes. . Some bacteria reproduce asexually by dividing every 20 minutes, and a single cell can producea billion descendantsin only l0 hours. . With this type of reproduction, a beneficial mutation can increase in frequency in a bacterial population very rapidly. . A bacterial cell with a mutant allele which makes it antibiotic resistant could produce an extremely large population of clones in a short period, while other cells without that allele are eliminated' . Although bacteria reproduce primarily by asexual means, most increase genetic variation by occasionally exchanging and recombining genes through processessuch as conjugation, transduction and transformation. 352 Unit IV Mechanisms of Evolution 2. Sexual recombination The contribution of mutations to genetic variation is negligible. ' Mutations are so infrequent at a single locus that they have little effect on genetic variation in a large gene pool. ' Although mutations produce new alleles, nearly all genetic variation in a population results from new combinations of alleles produced by sexual recombination. Gametesfrom each individual vary extensivelydue to crossingover and random segregationduring meiosis. . Thus, each zygote produced by a mating pair possessesa unique genetic makeup. . Sexual reproduction produces new combinations of old alleles each generation. Plants and animals depend almost entirely on sexual recombinationfor genetic variation which makes adaptationpossible. C. Diploidy and balanced polymorphism preserve variation Natural selectiontends to produce genetic uniformity in a population by eliminating unfavorable genotypes.This tendency is opposedby severalmechanismsthat preserve or restore variation. 1. Diploidy Diploidy hides much genetic variation from selectionby the presenceof recessive alleles in heterozygotes. . Sincerecessivealleles are not expressedin heterozygotes, less favorable or harmful alleles may persist in a population. . This variation is only exposedto selection when two heterozygotes mate and produce offspring homozygous for the recessive allele. ' If a recessiveallele has a frequency of 0.01 and its dominant counterpart 0.99, then 99% of the recessive allele copies will be protected in heterozygotes. Only l% of the recessive alleles will be present in homozygotesand exposedto selection. . The more rare the recessive allele, the greater its protection by heterozygosity. That is, a greater proportion are hidden in heterozygotes by the dominant allele. . This type of protection maintains a large pool of alleles which may be beneficial if conditions change. 2. Balanced polymorphism Selection may also preservevariation at some gene loci. Balanced polymorphism : The ability of natural selectionto maintain diversity in a population One mechanismby which selectionpreservesvariation is heterozygoteadvantage. . Natural selection will maintain two or more alleles at a locus if heterozygous individuals have a greater reproductive successthan any type of homozygote. . An example is the recessive allele that causes sickle-cell anemia in homozygotes.The locus involved codes for one chain of hemoglobin. . Homozygotes for this recessiveallele develop sickle-cell anemia which is often fatal. Chqta 23 TheEvolutionofPopulations353 . Heterozygotes are resistant to malaria. Heterozygotes thus have an advantagein tropical areaswhere malaria is prevalent, since homozygotes for the dominant allele are susceptibleto malaria and homozygous recessive individuals are incapacitatedby the sickle-cell condition. . In some African tribes from areaswhere malaria is common, 20Yo of the hemoglobin loci in the gene pool is occupiedby the recessiveallele. Other examplesof heterozygoteadvantageare found in crop plants (e.g., corn) where inbred lines become homozygous at more loci and show stunted growth and sensitivity to diseases. . Crossbreedingdifferent inbred varieties often produces hybrids which are more vigorous than the parent stocks. . This hybrid vigor is probably due to: l. Segregationof harmful recessivesthat were homozygous in the inbred varieties. 2. Heterozygoteadvantageat many loci in the hybrids. Balanced polymorphism can also result from patchy environments where different phenotypes are favored in different subregions of a populations geographic range (seeCampbell, Figure 23.9). Frequency-dependentselection also causesbalanced polymorphism (see Campbell, Figure 23.10). . In this situation the reproductivesuccessof any one morph declinesif that phenotype becomestoo common in the population. . For example, in Papilio dardanus, an African swallowtail butterfly, males have similar coloration but females occur in severalmorphs. . The female morphs resemble other butterfly specieswhich are noxious to bird predators. Papilio females are not noxious, but birds avoid them becausethey look like distastefulspecies. . This type of protective coloration (Batesian mimicry) would be less effective if all the females looked like the same noxious species,because birds would encounter good-tasting mimics as often as noxious butterflies and would not associatea particular color pattern with bad taste. 3. Neutral variation Some genetic variations found in populations confer no selective advantage or disadvintage. They have little or no impact on reproductive success.This type of variation is called neutral variation. . Much of the protein variation found by electrophoresis is adaptively neutral. . For example, 99 known mutations affect 71 of 146 amino acids in the beta hemoglobln chain in humans.Some, like the sickle-cell anemia allele, affect the reproductive potential of an individual, while others have no obvious effect. . The neutral theory of molecular evolution states than many variant alleles at a locus may confer no selective advantageor disadvantage. . Natural selection would not affect the relative frequencies of neutral variations. Frequencyof some neutral alleles will increasein the gene pool and others will decreasedue the chance effects of genetic drift. Variation in DNA which does not code for proteins may also be nonadaptive' . Most eukaryotes contain large amounts of DNA in their genomes which have no known function. Such noncoding DNA can be found in varying amountsin closely related species. 354 Unit IV Mechanismsof Evolution Some scientists speculate that noncoding DNA has resulted from the inherent capacity of DNA to replicate itself and has expanded to the tolerance limits of the each species.The entire genome could exist as a consequenceof being self-replicating rather than by providing an adaptive advantage to the organism. Transposonsmight fit this definition of "selfish DNA," although the degree of influence these sequenceshave on the evolution of genomes is not known. Evolutionary biologists continue to debate how much variation, or even whether "-'i-iil:nyilr"J#r;:,1f ff ,niffi::',l:;:ff,::':ilcesuchbe may occur in immeasurableways. Also, a variation may appear to be neutral under one set of environmental conditions and not neutral under other conditions. We cannot know how much genetic variation is neutral, but if even a small portion of a population's genetic variation significantly affects the organisms, there is still a tremendous amount of raw material for natural selection and adaptive evolution. [V. Natural Selection as the Mechanisms of Adaptive Evolution Adaptive evolution results from a combination of: . Chance events that produce new genetic variation (e.g., mutation and sexual recombination) . Natural selection that favors propagation of some variations over others A. Evolutionary fitness is the relative contribution an individual makes to the gene pool of the next generation Darwinian fitness is measuredby the relative contribution an individual makes to the gene pool of the next generation. . It is not a measureof physical and direct confrontation,but of the successof an organism in producing progeny. . Organisms may produce more progeny becausethey are more efficient feeders, attract more pollinators (as in our wildflowers), or avoid predators. Survival does not guaranteereproductive success,since a sterile organism may outlive fertile members of the population. r d long life span may increase fitness if the organism reproduces over a longer period of time (thus leaving more offspring) than other members of the population. . Even if all membersof a population have the same life span, those that mature early and thus have a longer reproductive time span, have increased their fitness. . Every aspectof survival and fecundity are componentsof fitness. Relative fitness: The contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus . For example, if pink flower plants (AA and Aa) in our wildflower population produce more offspring than white flower plants (aa), then AA and Aa genotypes have a higher relative fitness. Statistical estimates of fitness can be produced by the relative measure of selection against an inferior genotype. This measure is called the selection cofficient. . For comparison, relative fitness of the most fecund variant (AA or Aa in our wildflower population) is set at 1.0. Chrytt 23 TheEvolutionofPopuldions355 . . If white flower plants produce 80% as many progeny on average, then the white variant relative fitness is 0.8. The selectioncoefficient is the difference betweenthese two values (1.0 - 0.8: 0.2). . The more disadvantageousthe allele, the greater the selection coefficient. . Selectioncoefficients can range up to 1.0 for a lethal allele. The rate of decline in relative frequencies of deleterious alleles in a population depends on the magnitude of the selection coefficient working against it and whether the allele is dominant or recessiveto the more successfulallele. . Deleterious recessivesare normally protected from elimination by heterozygote protection. . Selection against harmful dominant alleles is faster since they are expressedin heterozygotes. The rate of increase in relative frequencies of beneficial alleles is also affected by whether it is a dominant or recessive. . New recessive mutations spread slowly in a population (even if beneficial) becauseselection can not act in its favor until the mutation is common enough for homozygotesto be Produced. . New dominant mutations that are beneficial increase in frequency faster since even heterozygotesbenefit from the allele's presence(for example, the mutant dark color producing allele in pepperedmoths). Most new mutations, whether harmful or beneficial, probably disappear from the gene pool early due to genetic drift. Selection acts on phenotypes, indirectly adapting a population to its environment by increasing or maintaining favorable genotypes in the gene pool. . Since it is the phenotype (physical traits, metabolism, physiology, and behavior) which is exposed to the environment, selection can only act indirectly on genotypes. The connection between genotype and phenotype may not be as simple and definite as with our wildflower population where pink was dominant to white. . Pleiotrop! (the ability of a gene to have multiple effects) often clouds this connection. The overill fitness of a genotype depends on whether its beneficial effects exceed any harmful effects on the organism's reproductive success. . polygenic traits also make it difficult to distinguish the phenotype-.genotype conniction. Whenever several loci influence the same characteristic, the members of the population will not fit into definite categories, but represent a continuum along a range. An organism is an integrated composite of many pherotypic features, and the fitness of a ginotype at any orie locus dependsupon the entire genetic context. A number of g"tt"J may work cooperatively to produce related phenotypic traits. B. The effect of selection on a varying characteristic can be stabilizing' directional, or diversifYing The frequency of a heritable characteristic in a population may be affected in one of three different ways by natural selection, depending on which phenotypes are favored (see Campbell, Figure 23.11) 1. Stabilizing selection Stabilizing selection favors intermediate variants by selecting against extreme phenotyPes. . The trend is toward reduced phenotypic variation and greater prevalence of phenotypes best suited to relatively stable environments' 356 Unit IV Mechanisms of Evolution ' For example,human binh weights are in the 3 - 4 kg range. Much smaller and much higher birth weight babies have a greater infant mortality. 2. Directional selection Directional selection favors variants of one extreme. It shifts the frequency curve for phenotypic variations in one direction toward rare variants which deviate from the average of that trait. ' This is most common when members of a speciesmigrate to a new habitat with different environmentalconditions or during periods of environmental change. . For example, fossils show the average size of European black bears increased after periods of glaciation, only to decrease during warrner interglacial periods. 3. Diversifying selection In diversifl,ing selection,opposite phenotypic extremesare favored over intermediate phenotypes. . This occurs when environmentalconditions are variable in such a way that extreme phenotypes are favored. . For example, balanced polymorphism of Papilio where butterflies with characteristics between two noxious model species (thus not favoring either) gain no advantagefrom their mimicry. C. Sexual selection may lead to pronounced secondary differences between the sexes Sexual dimorphism: Distinction betweenthe secondarysexual characteristicsof males and females . Often seen as differencesin size, plumage, lion manes, deer antlers, or other adomments in males. . In vertebratesit is usually the male that is the "showier" sex. . In some species, males use their secondary sexual characteristics in direct competition with other males to obtain female mates (especially where harem building is common). Thesemales may defeatother males in actual combat, but more often they use ritualized displays to discouragemale competitors. Darwin viewed sexual selection as a separateselection process leading to sexual dimorphism. . These enhanced secondary sexual characteristics usually have no adaptive advantage other than attracting mates. . However, if these adornments increasea males ability to attract more mates, his reproductive successis increasedand he contributes more to the gene pool of the next generation. The evolutionary outcome is usually a compromisebetweennatural selectionand sexual selection. . In some casesthe line betweenthesetwo types of selectionis not distinct, as in male deer. o I stag may use his antlers to defend himself from predators and also to attract females. Every time a female chooses a mate based particular phenotypic traits, she perpetuates the alleles that caused her to make that choice and allows the male with a particular phenotypeto perpetuatehis alleles (see Campbell, Figure 23.12). Chryter23 TheEvolutionofPopulations357 D. Natural selection cannot fashion perfect organisms Natural selectioncannot breed perfect organismsbecause: l. Organisms are locked into historical constrainrs. Each species has a history of descent with modification from ancestral forms. . Natural selection modifies existing structures and adapts them to new situations,it does not rebuild organisms. . For example, back problems suffered by some humans are in part due to the modification of a ikeleton and musculature from the anatomy of fourlegged ancestorswhich are not fully compatibleto upright posture. 2. Adaptations are often compromises. . Each organism must be versatile enoughto do many different things. . For example, sealsspend time in the water and on rocks; they could walk better with legs, but swim much better with flippers. . Prehensilehands and flexible limbs allow humans to be very versatile and athletic, but they also make us prone to sprains, torn ligaments,_and dislocations. Structural reinforcement would prevent many of these disablingoccurrencesbut would limit agility. 3' :' !::;i!'f,t|t'o::#:;'uff..t, to a rarge the genepooror popurations extent. 4 Alleles which become fixed in small populations formed by the founder effect may not be better suited for the environment than alleles that are eliminated. Similarly, small surviving populationsproducedby bottleneckeffect may be no better adapted to thJ environment or even less well adapted than the original population. :.'1ff:THH:HT'H:::fil::' characteristics New genes are not formed by mutation on demand' "better than" basis and subtle These limitations allow natural selectionto operateon a imperfections are the best evidence for evolution' ' REFERENCES 1998' N., et al.Biology.5thed.MenloPark,California:Benjamin/Cummings, Campbell, "Selfish Genes,the Phenotype Paradigm and Genome Doolittle, W.F. and C. Sapienza. 1980. 601-603, 284: Bvolution."Nature, edMettler, L.E., T.G. Gregg,and H.E. Schaffer.Population Geneticsond Evolution. Znd EnglewoodCiiffs, New Jersey:PrenticeHall, 1988' Sunderland,1982. Milkman, R. Perspectiveson Evolution.Sinauer,Massachusetts: